Chemistry Reference
In-Depth Information
as can be seen in the persistence of separate DSC melting patterns after intensive
melt mixing of relatively branched and unbranched versions of this polymer.
It is assumed in what follows that a satisfactory dispersion can be obtained
despite the problems that may be encountered in special cases, described above.
We consider the various procedures that may retard or eliminate the demixing of
such dispersions in the following section.
5.5.3
Procedures to Retard or Eliminate Demixing of Polymer
Mixtures
The various procedures that will be discussed are listed in
Table 5.5
in order to
present an overview of the basic ideas. Each heading in this table is considered
briefly in this section.
5.5.3.1
Use of Miscible Components
Thermodynamically stable mixtures will of course form stable blends. This
implies miscibility on a molecular level. It is desirable for some applications but
not for others, like rubber modification of glassy polymers.
1.
A particular polymer mixture can be made more miscible by reducing the
molecular weights of the components. From
Eq. (5-1)
any measure that
increases the entropy of mixing
Δ
S
m
will favor a more negative
Δ
G
m
. The
Flory
Huggins theory shows that the entropy gain on mixing a polymer is
inversely related to its number average size. This is observed in practice.
Low-molecular-weight polystyrenes and poly(methyl methacrylate) polymers
are miscible but the same species with molecular weights around those of
commercial molding grades (
100,000) are not.
Advantage can be taken of this enhanced stability of blends of low
molecular polymers by chain-extending or cross-linking the macromolecules
in such mixtures after they have been formed or applied to a substrate. This
procedure is the basis of many formulations in the coatings industry.
B
Table 5.5
Procedures to Retard or Eliminate Demixing
1. Use of miscible components (i.e.,
Δ
2. Rely on slow diffusion rates
(a) Mix high-molecular-weight
polymers
(b) Cocrystallization
0)
(a) Low-molecular-weight polymers
(b) Specific interactions to produce negative
Δ
G
m
5
Δ
H
m
T
Δ
S
m
2
#
H
m
(c) Generally match solubility parameters
3. Prevent segregation
(a) Cross-linking
(b) Forming interpenetrating networks
(c) Mechanical interlocking of components
4. Use
compatibilizing agents
“
”
(a) Statistical copolymers
(b) Graft copolymers
(c) Block copolymers
